When asleep the clock isn't running at all. You get a rather high 131uA in sleep mode which suggests you haven't powered down all the sections of the 328 (ADC for instance). Checkout Jeelabs.org and blog entries about low power mode - Jeelabs have a utility class called "Sleepy" in their Ports library which packages up all the power-saving and watchdog timer calls nicely - < 10uA should be achievable with just the watchdog timer running.
And back to the switching other loads issue - if they are all run off 5V or 3.3V then a p-channel MOSFET is a good solution. For higher voltage I'd think PNP transistor/darlington and a resistor + zener diode to level-shift the Arduino pin output to the base of the PNP.